Why are Iranian oil vessels vanishing at sea?

In 2014 an oil tanker coming from the United Arab Emirates (UAE) carrying diesel fuel disappeared from global tracking satellites in the Arabian Sea on its way to a Pakistani port. When its transponder—which signals the vessel’s location—was switched back on, it showed the ship had made a stop at a port in Iran, which was under international sanctions at the time.

Trading in Iranian oil, sold at a steep discount, could have invited stifling US sanctions on all the parties involved in the trade, prompting the banks to refuse payment for the shipment. The Pakistani middleman who arranged the cargo was arrested shortly after in Dubai as the banks feared they could be penalised and refused to clear his checks. But the matter was hushed up, and no investigations followed.

With the US sanctions targeting Iranian oil exports coming into force once again from November 5, vessels carrying Iranian oil have started to, once again, go dark in the open seas. Data analysts who keep track of shipping lines have noted dozens of instances where the transponders of the vessels had been switched off, making it difficult to ascertain where the Iranian oil is headed.

Such reports began emerging in September but by late October all the Iranian vessels had vanished from international........